drm/omap: dss: move platform_register_drivers() to dss.c and remove core.c

The core.c just for registering the drivers is kind of useless. Let's
get rid of it and register the dss drivers in dss.c.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930103840.18970-6-tomi.valkeinen@ti.com